1. Field of the Invention
The present invention relates generally to methods for the Non-Destructive Evaluation (NDE) of pipes and tubes using magnetostrictive sensor technologies. The present invention relates more specifically to an improved method for detecting defects in the signal analysis process associated with the use of magnetostrictive sensor technologies for the inspection of pipes and tubes.
2. Description of the Related Art
Magnetostrictive sensor technologies have been used successfully for a period of time with the inspection of pipes and tubes in processing plants such as refineries, chemical plants, steam process plants and the like. Examples of the use of magnetostrictive sensors, and the various analytical techniques associated therewith, are disclosed in U.S. Pat. Nos. 5,456,113 and 5,457,994, each entitled Non-Destructive Evaluation of Steel Cables and Ropes Using Magnetostrictively Induced Ultrasonic Waves and Magnetostrictively Detected Acoustic Emissions, as well as U.S. Pat. No. 5,581,037 entitled Non-Destructive Evaluation of Pipes and Tubes Using Magnetostrictive Sensors, all of which are commonly owned by the assignee of the present invention, Southwest Research Institute.
The techniques associated with such NDE inspections of pipes, tubes, cables and the like typically involve generating longitudinal waves along the length of the pipe or tube and analyzing signals that are reflected from defects and anomalies within the pipe or tube. One of the many advantages of this technique is the ability to detect defects by sensing the reflected signal at the same physical location at which the interrogating signal waves are generated.
Because mechanical waves generated by magnetostrictive sensors can propagate a long distance along a structure under inspection the techniques are capable of inspecting very long or large segments, typically more than a hundred feet under favorable conditions, of pipe very rapidly. These techniques also provide a complete volumetric inspection of a long section of pipe with minimum ancillary activity such as surface preparation, scaffolding or insulation removal. These magnetostrictive sensor methods therefore offer a very efficient and comprehensive mechanism for pipe and tube inspection.
In general, the longitudinal wave modes utilized in the above referenced techniques for inspection are dispersive in nature. This means that the velocity of the mechanical wave propagation varies with the wave frequency. FIG. 1 provides a illustrative example of the dispersion curves for the first two longitudinal wave modes, L(0,1) and L(0,2), which are typically used with the above referenced magnetostrictive sensor techniques. To simplify the detection of defects within the reflected signal pattern, the techniques described above utilize the waves in the frequency region where the dispersion curve is relatively flat (V.sub.o and V.sub.p) and avoid those regions (V.sub.o /2.pi.b and KV.sub.p /2.pi.b) where the dispersion curve changes rapidly with frequency. In order to confine the bandwidth of the wave pulse within the desired frequency range, and thus avoid excessive dispersion effects, a relatively narrow band excitation signal is used for transmitting the interrogating waves into the pipe structure. A tonal burst consisting of several cycles of sinusoidal wave at a specific wave frequency have been typically employed for this purpose. The effectiveness of magnetostrictive sensor techniques along these lines has been well proven in operating processing plants.
It has been recognized, however, that positive discrimination and identification of defect signals within the overall detected signal can sometimes be difficult due to the presence of extraneous signals not associated with defects and anomalies. These extraneous signals include those of other wave modes (mostly flexural) that might be produced in the pipe wall due to non-symmetric material properties and sensor excitation. In addition, for situations where the pipe or tube is liquid filled, extraneous signals can be caused by liquid induced dispersion effects. These extraneous signals may be confused as defect reflections or may mask or radically alter defect signals present in the detected signals.
Examples of previous attempts in the prior art to improve the defect detectability of NDE sensors include the following patents:
U.S. Pat. No. 5,612,495 issued to Shimada et al. on Mar. 18, 1997, entitled Non-Destructive Examination Device, describes a system that uses magnetostrictive transmitters and response sensors to carry out the non-destructive evaluation of a material. The system anticipates the use of a resonant frequency for the interrogating signal. The signal processing methods are described as potentially including a high speed Fourier transformation process or an integral process. No specific characterization or selection of the best or most appropriate signal processing method is made.
U.S. Pat. No. 5,526,689 issued to Coulter et al. on Jun. 18, 1996, entitled Acoustic Emission for Detection of Corrosion Under Insulation, describes a method and apparatus for detecting the presence of surface corrosion under insulation on a pipe structure. This patent anticipates the use of a broadband of acoustic waves to interrogate the structure. The sound waves are detected by piezoelectric sensors and converted to electrical signals for processing. The signal analysis method in Coulter et al. involves producing RMS voltage signals indicative of the detected sound waves and comparing the RMS voltage signals to standard signals obtained from uncorroded piping. The analysis involves a strict amplitude comparison to distinguish the signal component from the defect.
U.S. Pat. No. 5,195,046 issued to Gerardi et al. on Mar. 16, 1993, entitled Method and Apparatus for Structural Integrity Monitoring, describes a piezoelectric transducer based system designed for the detection, monitoring, and analysis of such things as aircraft structures. The system utilizes vibration signatures and recognizes changes in the vibration signatures as indicative of faults, cracks, deteriorations, etc. Various pattern recognition techniques are utilized. Data acquisition is accomplished using piezoelectric sensors and is digitized before being converted to the frequency domain via a fast Fourier transform. Time and/or frequency domain signatures are used in the signature pattern analysis. The patent lists 25 illustrative features (column 11) that include both time and frequency domain parameters as providing the basis for pattern recognition.
U.S. Pat. No. 5,665,913 issued to Chung on Sep. 9, 1997, entitled Method and Apparatus for Evaluation and Inspection of Composite-Repaired Structures, describes a system and method for NDE of composite-repaired structures wherein the signal transmitters and sensors are piezoelectric based devices. Analysis is carried out by comparing an output signal to a baseline reference signal generated at the time of composite repair. The signal processor involved in Chung includes an isolation filter, an amplifier, and a frequency domain integrator. The system anticipates the use of either a single frequency for interrogating the material or a range of frequencies.
U.S. Pat. No. 5,469,060 issued to Meyerand on Nov. 21, 1995 entitled Time Encoded Magnetic Resonance Imaging, describes a system that utilizes a separate set of signal transducers and applies a resonant frequency pulse to the material under investigation. The RF signals received as a function of time are converted to a set of frequency domain functions at specific times relating to specific strips in the image being generated. The frequency domain functions in the form of strips are combined sequentially to form the entire time frequency domain function image.
U.S. Pat. No. 5,719,791 issued to Neumeier et al. on Feb. 17, 1998, entitled Methods, Apparatus and Systems for Real Time Identification and Control of Modes of Oscillation, describes various methods for signal identification within a noisy background. The Neumeier et al. patent describes a system for controlling vibrations and oscillations with real time detection and compensation through an actuator. Various time frequency functions are applied to the signal for analysis.
U.S. Pat. No. 5,574,639 issued to Qian et al. on Nov. 12, 1996, entitled System and Method for Constructing Filters for Detecting Signals Whose Frequency Content Varies With Time, is generally directed to a signal analysis method intended to reliably detect the presence of signals of interest, especially those whose frequency content varies with time. The Qian et al. invention is directed to the development and use of time templates designed to match the received signal of interest. The patent describes its application as including any situation where a signal travels through media and wherein the media transforms or disperses the signal into a signal whose frequency content varies with time. The patent applies what is referred to as the Gabor Spectrogram to a computer representation of the signals in the joint time-frequency domain for the purpose for instantaneous frequency estimation. The patent distinguishes STFT as not being the ideal representation for tracking the parameters of concern.
U.S. Pat. No. 5,439,157 issued to Geier et al. on Aug. 8, 1995, entitled Automated Butt Weld Inspection System, describes an electromagnetic acoustic transducer (EMAT) based system that produces an ultrasonic shear wave in a material containing welds and the like. A second EMAT receives the reflected shear wave.
None of the above described methods and systems, however, have been able to adequately distinguish signal characteristics that derive from extraneous features of no interest from those signal components that derive from the types of defects and anomalies of interest in a damage, deterioration, or corrosion study. The signal analysis techniques utilized thus far are generally incapable of distinguishing a time, frequency and amplitude signal component associated with a defect of interest from similar time, frequency and amplitude signal components associated with geometric and/or otherwise non-relevant causes.
It would be desirable to have an NDE system for pipe and/or tube inspection that utilizes magnetostrictive sensor techniques and is capable of analyzing a detected signal in a manner that permits the discrimination of extraneous signal components not representative of defects or anomalies of interest. It would be desirable if such a system could be automated to the extent that rapid discrimination of such signal components not representative of defects or anomalies of interest, would provide an accurate and rapid identification of the location and nature of defects and anomalies of interest.